A light control device comprising a polymer/liquid crystal composite material comprising a polymer material typified by an acrylic acid-based polymer as a matrix with a liquid crystal material dispersed therein does not need a polarizing plate or an oriented film and can switch between a transparent state and an opaque (cloudy) state by turning on/off an electric field, unlike a conventional display comprising a liquid crystal. Therefore, it is possible to provide a light control device having a larger area, and application to building materials such as window glass and partitions is expected. Various polymer/liquid crystal materials that can constitute light control devices have been proposed [for example, Japanese Patent Laid-Open No. 2-55318 (Patent Literature 1), U.S. Pat. No. 4,435,407 (Patent Literature 2), Japanese Patent Laid-Open No. 2003-155388 (Patent Literature 3), and Japanese Patent Laid-Open No. 2008-266633 (Patent Literature 4)].
Japanese Patent No. 4630954 (Patent Literature 5) discloses a polymer/liquid crystal composite material obtained from a polymerizable composition comprising acrylic monomers that are a methacrylate acid ester and an ethoxylated trimethylolpropane triacrylate ester, and a urethane-based diacrylate soft oligomer represented by the formula R1—(R2—R3)n═R2—R1, wherein R1 is an acrylic group, R2 is an isocyanate residue, and R3 is a diol group. Patent Literature 5 purports that the polymer/liquid crystal composite material has high viscosity that enables the composite material to form a uniform composite film and high tackiness to a base material and is useful for light control glass.
The polymer/liquid crystal material described in Patent Literature 5 has several practical problems as shown below because it is a liquid crystal material comprising a urethane-based diacrylate soft oligomer.
For example, the tensile modulus of a urethane-based material is 70 to 690 Mpa. Therefore, it can be said that the urethane-based material is softer and more pliable in terms of physical properties than an acrylic material, which has a tensile modulus of 2200 to 3200 Mpa. But such a low tensile modulus is a fatal drawback in a polymer/liquid crystal composite material (polymer/liquid crystal composite film) used for a large light control device and the like, and thus causes local unevenness and partial transparency due to stress and bending. In other words, the polymer is likely to deform due to external stress, and the liquid crystal aligns in a fixed direction following the deformed polymer to thereby locally occur unevenness in which the transmittance changes and transparency.
In addition, it is necessary to remove a polymer during the formation of the electrodes of a light control device. However, when in using the polymer/liquid crystal composite material described in Patent Literature 5, indium tin oxide (“ITO”) and the polymer adhere to each other more than necessary at the interface therebetween, and therefore time is required for the cleaning of the ITO surfaces, which is problematic. Further, for the polymer/liquid crystal composite material comprising a urethane-based acrylate and the like as described in Patent Literature 5, it is necessary to increase the cell gap (generally 18μ) in order to sufficiently ensure shielding properties when the electric field is turned off, and as a result, the viewing angle when the electric field is turned on is very poor.
Further, the polymer/liquid crystal composite material comprising a urethane-based acrylate and the like in Patent Literature 5 is very sensitive to UV polymerization temperature during film formation in the step of forming a polymer network by UV polymerization. Therefore, another problem is that unless the temperature control is precise during film formation, the driving characteristics of the resulting light control device vary to thereby provide transparency deterioration when the electric field is turned on and unevenness when the electric field is turned off.
In order to solve above-described various problems of such a polymer/liquid crystal composite material comprising a urethane-based acrylate and the like, including the tensile modulus and the transmittance, the present inventors have been proposed a polymer/liquid crystal composite material comprising an acrylic monomer and the like [for example, Japanese Patent No. 5386630 (Patent Literature 6)].
The polymer/liquid crystal material described in Patent Literature 6 is a new polymer/liquid crystal material, but a problem thereof is that when a light control device is manufactured using it, the adhesive strength between the layer of the liquid crystal material comprising the acrylic monomer and the ITO layer tends to weaken. Therefore, further improvement in the adhesive strength between these layers is desired in order to endure practical use as the light control device.